The present invention relates generally to vacuum columns used in oil distillation, and more particularly to the problem of erosion in the vapor horn section of the column.
In petroleum refining, a vacuum column is used to distill feed stock at reduced pressure and high temperature to recover additional distillates (such as vacuum gas oils, lubricating oils, and/or conversion feedstocks) from reduced crude, which is the bottoms product of an atmospheric distillation unit.
The feed from an atmospheric distillation unit enters a “vapor horn” section of the vacuum column through a feed inlet. The feed inlet can be either tangential or radial. An internal box or tangential distributor assists in dispersing the feed. In general, the temperature in the vapor horn section may approach 800° F. (420° C.) at a pressure in the range of about 7.5 to 14.7 psig external pressure.
Conventionally, the shell of the vacuum column in this section of the column is protected against erosion by a stainless steel liner plate attached to the inside of the vessel. The liner plate may be about 6 feet high and commonly subtends around 270 degrees of the circumference of the column.
The liner plate generally is welded to the shell of the vacuum column at the perimeter of the liner plate. The shell of the vacuum column is usually composed of clad plates that each have a stainless steel clad interior surface (up to ⅛″ thick or so for conosion resistance) on a carbon steel backing plate. Generally, the liner plate is perimeter welded to the carbon steel backing plate after stripping sections of the stainless steel clad interior surface from the backing plate. To provide an adequate bond between the liner plate and the shell of the vacuum column, the liner plate is also commonly plug welded to the shell at about 12″ to 15″ intervals. The plug welds are commonly welded directly to the stainless steel cladding on the shell plates.
The installation of such liners is time-consuming and costly. Further, cracks can develop at the plug weld locations, requiring expensive downtime to repair and replace the liner plate.
Another known procedure for providing additional erosion resistance in the vapor horn section of a vacuum column involves adding a weld overlay to the inside surface of the column in that section. This procedure is generally viewed as uneconomical, and could cause local shrinkage or distortion.